Acceleration Problem with 07' Prius

Hello, I'm a first time poster who just came across this forum and really hope someone can help me out with an issue I just encountered about 10 days ago with my 2007 Prius. My Prius has 35,000 miles and is about 15 months old, but last Monday I filled up with gas before heading to work and immediately noticed a severe lack in acceleration power when merging on to the freeway. Ever since then my car never down-shifts while accelerating. It takes about twice as long just to get the car to 40 mph as it did previously. If I'm trying to merge on a freeway from a stop light it will constantly accelerate all the way to 70 or 80 mph with no automatic down shift. Naturally there's been a total noticeable loss in gas mileage. I live in Southern California so there shouldn't be any weather-influenced issues. There hasn't been a warning light or anything that's popped up, my car just hasn't been fun to drive since that day because it seems like it's really struggling to accelerate in stop and go traffic and freeway driving. I took it to my local Toyota dealer and they said they ran a diagnostic and found nothing, but it's still giving me problems. Any ideas?

 

Yeah, sorry...I meant to describe it as feeling AS IF it wouldn't down-shift, like it was almost struggling to do something it could't. But let me ask you if you think that the gas I was using could have contributed to the problem. The only reason I ask is because it seemed like the issue presented itself immediately after filling up at my local AM PM. About a week later I filled up at the same AM PM location and still had my issue. On my way to school tonight I filled up at Chevron instead and my car seems to be running with it's old strength again. Is this just a figment of my imagination or could gas actually have that kind of affect on my acceleration?